On November 12th, Nashville will host Green Roofs for Healthy Cities, an event that brings together vendors and green roofing advocates to promote the integration of green roofing throughout North America. And, as if on cue, this year’s event will be hosted at the Music City Center, a LEED platinum building which sports one of the largest green roofs in the country.

At 170,000 sq ft, the green roof at Nashville’s new convention center is designed to stem the peak rainwater flow from surging into the combined sewers under Nashville.

The Music City Center will host “Green Roofs for Healthy Cities”. This is the MCC’s Green Roof on a shady day.

What is Green Roofing?

Green roofing, or living roofs, help curb ground water pollution by reducing the volume of water run-off (and subsequent pollution from materials on the ground that are picked up by flowing water) that ends up in the sewer. Green roofs not only help reduce run-off, but they also help cities scale sewage systems for future growth in a more economic way.

To make a green roof, soil is poured and plant on your roof. Long practiced in Europe, the modern day application over bituminous and single ply roofing material can benefit all of us supposedly.

So if it such a good idea, why aren’t there more green roofs in Nashville? In this article, the Nashville Green Magazine will examine the benefits, weigh them against the costs and see if we can determine why green roofing is not as prevalent in Nashville and indeed greater parts of the southern US compared to our Northern cities where it seems to be commonplace on public buildings.

One proposed benefit is that by covering our buildings with plants, we soften the built up landscape, thus reducing the urban heat island effect (UHI), the process of our built up urban environment absorbing UV radiation from the sun and re-emitting it slowly overnight. The result of the UHI is it raises our urban ambient temperatures and makes it more expensive to air condition our homes. Plus gardens look better than roofing.

Another benefit of green roofing is that it will extend the life of roofing materials by shielding them from UV and ‘ thermal flexation’; the rapid expansion and contraction of roofing material exposed directly to sunlight. Data from buildings that have green roofs in Germany, the longest collection of data for modern construction, show that roofing can last as much as 50% longer when covered with vegetation. So aside from aesthetics and the opportunity to use the space on top of the building, there is a financially tangible reason for installing a garden on your roof.

Stormwater Runoff and Combined Sewer Overflows

The primary reason for green roofing is the management of stormwater runoff. The single greatest source of our fresh water pollution comes from Combined Sewer Overflows (CSO’s for short). Standby for some heady info on sewage. CSO’s is the term given to the rate of expanded sewage flow that runs through storm plumbing to our municipal waste treatment plants when it rains. The simple fact of sewage processing is that municipalities are designed to treat sewage at the rate of households (with that toilets and drains) per lineal mile of sewer plumbing. The older our community, the more the sewage mixes with runoff from streets and roofing as older methods of urban development made no separation of these independent sources. Urban development and municipal waste treatment systems have only recently separated the flows, yet the volume of runoff has exploded as the area of impervious surfaces in our built environments has grown to accommodate automobile centered urban and suburban development.

Municipal waste treatment can only process a steady amount of sewage input. When a storm inflates the runoff, municipalities have no choice but to close the doors and let the expanded flow, although diluted, run directly and untreated into our streams, rivers and lakes. This is why beaches are closed in the summer after big storms when the measurable e.coli and other pathogens from sewage exceeds safe levels.

Since the passing of the Clean Water Act in 1974, the EPA has had the onerous responsibility of monitoring the incidences of such pollution and the Federal and State governments have had the responsibility for investing in waste treatment to keep up with increased development and waste treatment demands. Long story short, we are not really keeping up. It is more expensive to dig up the sewers and separate the flow of storm drains from sewer pipes than we can justifiably imagine.

According to the EPA (Environmental Protection Agency), there are about 720 communities in the US that suffer from CSO’s – and Nashville is one of them.

The frequency and severity of CSO events is also strongly influenced by climatic factors governing the occurrence of urban stormwater runoff, particularly the form (i.e., rain or snow), the amount, and the intensity of precipitation. If climate change continues to impact the incidences of storm size and frequency the challenge for our cities will increase. Municipalities are somewhat restricted in how they can respond to increased demands for sewage treatment and costs are going to increase.

So the next step is to make the developers and owners of buildings in our built environments responsible for the runoff caused by construction of impermeable surfaces. Nashville like other cities has tried to incentivize the construction of green roofing in the area of experience.

Experience in the Green Roofing Industry

First, allow me (the author) to establish some credibility on the topic. I ran a manufacturing company which sold branded green roof components to roofing manufacturers who were looking for assemblies to sell to clients seeking to put a green badge on their new facilities. I was responsible for the manufacture and even some of the installation of green roof systems on over 1,000,000 sf of commercial green roof infrastructure, including the FedEx Cargo Sort building at O’hare Airport, The American Life Building in Louisville, Ky, and the Brighton Park Public School in Chicago. All of these projects were undertaken for different reasons and only one achieved a respectable ROI from an operations standpoint. So let’s review the cost and return from an investment point of view and use this to evaluate incentives meant to encourage green/vegetative roofing.

Cost of Green Roofing

The cost of green roofing is to be estimated to be between $14 to over $20 per sq ft installed. If you know anything about roofing, that is double to triple the cost of a new roof conservatively. The green roofing is expensive in the US. Growing media is comprised primarily of expanded aggregate typically used for lightweight concrete but when sold to the landscape industry, the cost rises to $120 per cubic yard. Add transport and crane cost for hauling it to the roof level and media becomes 25% of the cost of your green roof. The plants are by far the most expensive cost. Green roof plants described below, will cost $0.75 to $2.50 per plug. You will need over 6 plugs per square foot to give a decent coverage of plants. So plants can cost you half of the total expense.

Green rooftop of the Pinnacle Building near Nashville’s waterfront.

Return on Investment for Green Roofing

1. Roof insulation? Energy returns have been researched and are nominal given the advances in roof insulation products and techniques so we can remove any measurable UHI benefit for the building with the green roof. Besides, the R value of frozen soil in the winter is negligible.

2. Increasing the lifespan of your roof is definitely quantifiable but given the immediate outlay, ( i.e. time value of money) and the likelihood that any owner will most likely not retain ownership of any property to receive the benefit, quantifying the value in a sale may be difficult. Most investors fear the impact of a green roof if a leak develops so will most likely view a green roof as a liability.

3. The aesthetics of a green amenity space on a building for the tenants use has been shown to increase rental values for the building owner. The Chicago Chamber of Commerce did a study after the greenroof mandate and determined that green roof amenity spaces were instrumental in helping owners raise rental rates where the space was available to the tenants. These types of amenities also fall under plaza and concourse developments that add the potential for retail development in downtown commercial areas.

White roofing has a smaller impact on ambient temperature in urban settings.

Incidentally, amenity type plazas with green roofing contain large areas of pavers for people to congregate so this type of ‘greenroof’ is not meant primarily to detain runoff. It looks much better from above and is certainly better than looking out your window at high albedo roofing cluttered with HVAC units.

For those of you not in the roofing lingo, high albedo roofing materials is the newest development of roofing membranes that are made of white, UV reflecting material instead of the historically black, high carbon, asphalt based roofing material. Obviously, white roofing reflects energy and black roofing absorbs UV radiation and energy. White roofing has been widely deployed since the late 90’s to reduce UHI effects in urban environments and economically, it is much more effective for reducing the temperatures on rooftops than green ‘vegetative’ roofing. The added premium for cooling roofing alternatives are less than $0.30 per square foot. We can now logically discount the value of fighting UHI effects with green roofing.

Low maintenance, sedum plants do not require mowing!

Green roofing companies generally sell a specific plant variety for use in the lowest cost type of green roof; the Extensive system. This type of green roof is generally under 6 inches in media depth, the lightest weight burden for a roof to support. As the growing media is so shallow, only succulent type plants (with shallow root systems) are suited to grow in this environment. Called Sedum or stonecrop, these low growing plants are drought-resistant – ideal for the roofing environment. In the best environments, sedum plantings on a green roof are ideal. They cover completely as long as the roof has significant sun exposure (at least 6 hours per day). Green roofs do not require mowing and even remain green in the winter as some of the varieties only bloom in late fall when temperatures approach freezing. These plants originate in high altitude environments and grow in rocky outcroppings naturally. They require cool nights and generally grow best in autumn and spring as a result of their preference for cool temps.

The lush palette of green creates an expectation of a low maintenance, carpet like vegetative cover for an area most people never see. Perfect right?

The reality is in hot humid climates, these types of plants are very challenged. In fact only two of the over 600 the crassulaceae genus of stonecrops plants successfully grow in USDA zones 7 and 8. Very few roofing situations have the solar exposure of the MCC so this should be a perfect environment – except for the humidity of Nashville. In order to make them work on green roofs like at the Music City Center, which has a media depth of only 2.5”, the facility has to irrigate it to keep the plants cool and alive on very hot days. This has clearly worked for the sedum so far but re plantings have been significant and weeds are a constant menace in the moist media. Invasive species grow much faster than sedum ground cover, and the contract to keep weeds at bay is not cheap. So much for the ‘low maintenance’ idea. The American Life Insurance building in Louisville also had irrigation installed. See below.

American Life Insurance Building in Louisville, KY.

Green roofing that requires irrigation to survive is not an economical green roof. The idea is to prevent water from rushing into the sewers during a storm event. Most high flow events happen in the hot humid weather. If the green roof media is already saturated from irrigation, how much storm water can be detained?

The Music City Center green roof is not connected to sewers however. The practice of new LEED qualified construction is to separate roof and plaza surfaces to cisterns saving water for landscape irrigation. MetroWater is actively changing standards to alleviate the conditions that cause runoff pollution so unlike Chicago, which had a major freshwater problem developing, Nashville may be ahead of the CSO pollution problem getting worse from stormwater runoff as it enters the next phase of growth. The biggest danger from runoff may be sewage from the smaller municipal waste facilities like Harpeth Valley Utility District serving Bellevue, Berry’s Chapel Utility Inc.and Cartwright Creek LLC, serving Franklin in Williamson County.

Perhaps the Nashville Metro Water incentive should be extended to newer developments in the ring communities as well. A larger portion of new development here will contain extensive parking because these communities are dependent on vehicular travel. The city may find that development partnerships can help the protection of smaller rivers like the Harpeth, Stones river and Duck river too.